Patent Document 1 (Unexamined Japanese Patent Application Publication No. 2005-352378) discloses an area division type wave plate. The area division type wave plate is a wave plate that changes the state of polarization of light, in which dielectric flat plates are arranged with a microscopic period to form a fine-periodic structure. In one base member of the wave plate, there are two or more areas which differ from each other in at least one of a thickness of the dielectric flat plates, a height of the dielectric flat plates, and a period of the arranged dielectric flat plates. Further, in the areas, two or more areas differ from each other in a direction of an optical axis.
Further, Non-patent Document 1 (Sholiro Kawakami, “At the front of practical application of photonic crystal”, O plus E Vol. 28, No. 4, April 2006) discloses wave plates for reading information from a multi-layer optical disc, as an application of the area division type wave plate employing a three-dimensional periodic structure (photonic crystal). Each of the wave plates 99 and 100 includes periodic structures shown in FIG. 15(a) and is equipped with areas 101 and 102 divided by parting line (boundary) 103. These wave plates 99 and 100 are arranged in an optical system for optical pickup shown in FIG. 15(b). Namely, two wave plates 99 and 100 each having divided areas 101 and 102 are arranged as shown in FIG. 15(b) to change polarization by 90° between a light beam (solid line) that focuses at the point between the wave plate 99 and wave plate 100 and a light beam (broken line) that focuses at the point on the outside of the wave plate 99 and wave plate 100. Thereby, the light beams are separated by polarizing plate 104. Since the polarizing plate 104 transmits the light beam shown with a solid line, it transmits only reflected light coming from a position at a certain depth in the multi-layer optical disc as shown in FIG. 15(b).